This application relates to automatic control of digital cameras and other electronic digital image acquisition devices, and the use of visual data generated by them, in a manner that selectively increases the dynamic range of the cameras or other devices.
Electronic cameras image scenes onto a two-dimensional sensor such as a charge-coupled-device (CCD), a complementary metal-on-silicon (CMOS) device or other type of light sensor. These devices include a large number of photo-detectors (typically two, three, four or more million) arranged across a small two dimensional surface that individually generate a signal proportional to the intensity of light or other optical radiation (including infrared and ultra-violet regions of the spectrum adjacent the visible light wavelengths) striking the element. These elements, forming pixels of an image, are typically scanned in a raster pattern to generate an analog signal with a time varying magnitude representative of the intensity of radiation striking one sensor element after another as they are scanned. Color data are most commonly obtained by using photo-detectors that are sensitive to each of distinct color components (such as red, green and blue), alternately distributed across the sensor.
A popular form of such an electronic camera is a small hand-held digital camera that records data of a large number of picture frames either as still photograph “snapshots” or as sequences of frames forming a moving picture. A significant amount of image processing is typically performed on the data of each frame within the camera before storing on a removable non-volatile memory such as a magnetic tape cartridge, a flash memory card, a recordable optical disc or a removable hard disk drive. The processed data are typically displayed as a reduced resolution image on a liquid crystal display (LCD) device on the outside of the camera. The processed data are also typically compressed before storage in the non-volatile memory in order to reduce the amount of storage capacity that is taken by the data for each picture frame.
The data acquired by the image sensor are typically processed to compensate for imperfections of the camera and to generally improve the quality of the image obtainable from the data. The correction for any defective pixel photodetector elements of the sensor is one processing function. Another is white balance correction wherein the relative magnitudes of different pixels of the primary colors are set to represent white. This processing may also include de-mosaicing the individual pixel data, when obtained from a type of sensor having spatially separate monochromatic pixel detectors, in order to render superimposed multi-colored pixels in the image data. This de-mosaicing then makes it desirable to process the data to enhance and smooth edges of the image. Compensation of the image data for noise and variations of the camera optical system across the image and for variations among the sensor photodetectors is also typically performed within the camera. Other processing typically includes one or more of gamma correction, contrast stretching, chrominance filtering and the like.
Electronic cameras also nearly always include an automatic exposure control capability that sets the exposure time, size of its aperture opening and analog electronic gain of the sensor to result in the luminescence of the image or succession of images being at a certain level based upon calibrations for the sensor being used and user preferences. These exposure parameters are calculated in advance of the picture being taken, and then used to control the camera during acquisition of the image data. For a scene with a particular level of illumination, a decrease in the exposure time is made up by increasing the size of the aperture or the gain of the sensor, or both, in order to obtain the data within a certain luminescence range. An increased aperture results in an image with a reduced depth of field and increased optical blur, and increasing the gain causes the noise within the image to increase. Conversely, when the exposure time can be increased, such as when the scene is brightly lighted, the aperture and/or gain are reduced, which results in the image having a greater depth of field and/or reduced noise. In addition to analog gain being adjusted, or in place of it, the digital gain of an image is often adjusted after the data have been captured.
Digital camera systems typically have a limited dynamic range. While film cameras typically provide images with intensity dynamic ranges ranging from 70 dB to 80 dB, current digital cameras typically offer dynamic ranges of less than 60 dB. One cause of this is the use of digital processing paths within the cameras that are limited in the number of bits they can carry. The cost of a digital camera can increase rapidly as the width of digital processing paths is increased. As the number of bits of data that an analog-to-digital converter or digital signal processor can process increases, as an example, the cost of a camera can increase significantly. The dynamic range of digital cameras is therefore typically maintained lower than that which can be obtained by film cameras in order to be cost competitive.
This often results in one or more regions of an image having a wide intensity dynamic range being saturated, either positively or negatively, or both. Any details of the image in saturated regions are lost since these regions are uniformly bright or dark, respectively.